The invention relates generally to fuel cells, and more particularly to flow field plates for fuel cells having surface treated carbon coatings thereon.
Electrochemical conversion cells, commonly referred to as fuel cells, produce electrical energy by processing reactants, for example, through the oxidation and reduction of hydrogen and oxygen. A typical polymer electrolyte fuel cell comprises a polymer membrane (e.g., a proton exchange membrane (PEM)) with catalyst layers on both sides. The catalyst coated PEM is positioned between a pair of gas diffusion media layers, and a cathode plate and an anode plate (or bipolar plates) are placed outside the gas diffusion media layers. The components are compressed to form the fuel cell.
The bipolar plates act as current collectors for the anode and cathode. They have channels and opening for distributing the fuel cell's gaseous reactants over the surface of the anode or cathode catalysts. (The plates will be referred to as bipolar plates hereinafter, but they could also be unipolar anode or cathode plates as is understood by those of skill in the art.)
Bipolar plates for fuel cells need to be corrosion resistant, and electrically conductive (e.g., a resistance of less than about 20 mΩ·cm2). Desirably, they should also be low cost. The most commonly used bipolar plates currently in use are made of stainless steel. Stainless steel is advantageous because of its mechanical strength when made into thin sheets. However, untreated stainless steel has a passive oxide film which has high resistance (about 250 mΩ·cm2). Consequently, the oxide coating needs to be removed, and the stainless steel coated with a conductive coating, such as a gold coating or a polymeric carbon coating, in order to be used in fuel cells. These coatings generally require expensive equipment to deposit, which adds to the cost of the finished plate.
Currently, gold is frequently used as the conductive coating on bipolar plates. In some cases, the coating is approximately 3-5 nm thick. The gold coating provides low contact resistance (less than about 20 mΩ·cm2). However, the high cost of gold makes the use of a gold coating in a fuel cell undesirable. In addition, it is difficult to make a defect-free coating because the coating is so thin. Defects in the gold coating create islands of exposed stainless steel. Over time, the air in the system reacts with the exposed stainless steel, and an oxide layer of about 1-3 nm can build up on the surface. The oxide layer formed by this competing reaction can cover the gold coating, rendering the surface with high contact resistance.
In addition, it would be advantageous for part of the coating to be hydrophilic while another part is hydrophobic. Most of the plate material is desirably hydrophilic to assist with stability issues at lower power. A hydrophilic coating helps to remove water from the bipolar plate. However, the exit manifold is desirably hydrophobic so that the water does not stick as it is exiting the bipolar plate and block the exit manifolds, which would result in back flow of water.
Therefore, there is a need for a low cost, corrosion resistant, electrically conductive coating with desired surface properties for use on bipolar plates in fuel cells.